We welcome to SAGANet, Dr. Darlene Lim, a geobiologist based at the NASA Ames Research Center, who is actively involved in the development of operational concepts for human scientific exploration of our solar system. She is currently the PI of BASALT (Biologic Analog Science Associated with Lava Terrains), PI of the Pavilion Lake Research Project, and Deputy PI of FINESSE (Field Investigations to Enable Solar System Science and Exploration).

Why is evolution and adaptations of life important and relevant to astronomy?

Based on our current understanding of life on the Earth, we can say with a great degree of confidence that life’s origin and evolution occurred in a very limited range of physical conditions for a period of a few billion years. Therefore, it is natural to probe the properties of astronomical objects governing the physical conditions leading to the origin and evolution of life on Earth. The search for habitable extraterrestrial environments has generated tremendous interest in the study of planets orbiting stars other than our Sun, or extrasolar planets (exoplanets in short). Such studies are helping us to understand the birth and evolution of stellar systems in our Galaxy and also leading to new developments in astronomical instrumentation. The search for evidence of habitable environments on Mars has lead to a number of scientific missions to the planet which provide insights into the history of the Solar System, amongst other things.

How does prebiotic chemistry help us to understand how life started?

Let’s start by defining prebiotic chemistry as chemical reactions that happened before the emergence of life as we know it. There are many theories for the origins of life on Earth, but we have yet to connect all the dots to complete the story. One theory as to how life originated on our planet points to an “RNA World” where molecules of the genetic polymer RNA (Ribo Nucleic Acid – similar to DNA, Deoxyribo Nucleic Acid) formed. According to this theory, these molecules of RNA had the ability to copy themselves and were later encapsulated by lipids (certain types of organic molecules) to form what we call “proto cells.”

Here we ask the question: where did the RNA come from in the first place? To answer this question, we study the structure of nucleotides, the building blocks of RNA. A nucleotide has three components: a sugar (ribose), a phosphate, and a heterocycle (a certain type of organic compound). Let’s focus on one of these components, ribose (a molecule of this sugar has five carbon atoms).

Chemists are interested in what kind of molecules might have been present on the early Earth or in interstellar space, billions of years ago, to assess what chemical building blocks for life might have been available to start with. To gain a better understanding of what molecules can be added to my toolbox of prebiotic chemistry, we collaborate. We ask fellow scientists – astrochemists, geochemists & geologists – who have a better understanding of what is out in deep space and what may have been on the early Earth, when the planet cooled just enough to enable water to be liquid on its surface.

For example, astrochemists have confirmed the presence of simple organic molecules such as formaldehyde, glycolaldehyde, and glyceraldehyde in interstellar gas clouds. These three molecules contain one, two and three atoms of carbon respectively. Using chemistry, we can mix these molecules in the lab in various ratios, at different temperatures, and at varying pH levels, and we can include a variety of elements or minerals that we think were present on an early Earth. One prebiotic chemistry experiment, for example, showed that we could mix these simple organic molecules with borate to form stable sugars including the five-carbon ribose. Another molecule present in interstellar gas clouds is cyanide, which can undergo a chemical reaction to evolve into the heterocyclic molecule adenine. Here we now have two pieces of a nucleotide. The next step is to understand how these molecules join together to form the backbone or RNA.

One perk of being an astrobiologist is the opportunity to collaborate with scientists in other fields and the perpetual learning we all engage in while we answer some of the most important questions relating to our own existence.

Life Detection

Could scientists in a lab recreate the conditions of alkaline hydrothermal vents (similar to what we think first life on Earth formed in) to see if they could recreate life?

Scientists are doing exactly what you suggest! They are simulating in the laboratory the conditions believed to have been present at hydrothermal vents on the ancient Earth. They are doing this to test the hypothesis that life could have originated at hydrothermal vents when Earth was young (~4 billion years ago). The laboratory conditions don’t have to be extremely sterile, since the chemicals produced under these conditions are not generally ones that would be immediately used by modern life. The scientists (for example, at the Jet Propulsion Laboratory in Pasadena, California) have found that biologically important compounds such as amino acids and peptides are produced under ancient hydrothermal vent conditions. Further, when the simulated vents form minerals like those in the chimneys we see in modern hydrothermal systems, the minerals contain microcompartments that could have separated a prebiological chemical mixture from its environment, in essence forming the first proto-cell. So this continues to be an active area of research, and one of many alternatives that are being studied to help us understand the origin of life on Earth.

Is it possible to build a tricorder that can detect life on other planets, using a portable system. If so, how would you do it - laser spectroscopy, lab on a chip, etc.?

A “tricorder”, as depicted in “Star Trek,” is a multi-purpose technological device used by human explorers. Present-day human space exploration has not yet taken any astronaut further than the Moon, although some initial planning for human missions to Mars is in progress. Only when humans are able to set foot on the surface of another planet will there be a need for a mobile tricorder-like device that explorers can use to detect life as they explore other planets.

That said, the technology that might go into such a device has real-life analogues with scientific instruments fitted to robotic planetary exploration rovers. The current robotic Mars Exploration Laboratory mission, for example, includes a search for chemical signatures in rocks or soil that could indicate that Mars is or was habitable to present or past life. This roving lab is outfitted with drills and scoops to collect samples, which are then fed into chambers that allow for the composition of the material to be analyzed according to its mass (using a mass spectrometer), its volatiles (using gas chromotography) or its laser-induced signature (using a laser spectrometer). The Mars Science Laboratory (named “Curiosity”, see: http://mars.jpl.nasa.gov/msl/) even includes an instrument that shoots a laser at a distant rock and observes its chemical composition from afar! Scientists compare the results of these chemical analyses with their knowledge of environments on Earth to understand whether or not these Martian environments are consistent with the presence of biology (for example, by looking for organics, carbon, amino acids, or other remnants of life). Although we cannot predict what technology in the future will look like, we can at least be sure that the techniques used in the present-day robotic exploration of Mars will be available for future human exploration of the Solar System.

Some people say that aliens are real. Are UFOs real?

Scientists have not yet found any evidence of extraterrestrial life, or what some people refer to as “aliens.” Scientists are studying possible habitable environments in our Solar System in hopes of finding such evidence. Two places where scientists think life may have existed or may exist today are the subsurface of Mars and the ice-covered liquid water ocean on Europa (a moon of Jupiter).

Astronomers have detected hundreds of “extrasolar planets” — that is, planets orbiting stars other than our Sun. While scientists do not yet have the ability to detect evidence of life on extrasolar planets, they are able to speculate on whether those planets might be habitable to life as we know it.

As to UFOs, what “UFO” stands for is “unidentified flying object.” Some people seem to think “UFO” means “unidentified flying object from another planet.” Certainly a person might see objects that appear to be flying and that no one can identify. However, scientists have not found any evidence that extraterrestrial beings or vehicles have visited Earth.

If you are interested in learning about the history of UFO sightings, you may wish to read a report published by the Congressional Research Service in 1983. You can find this report online at: http://www.nicap.org/docs/TheUFOEnigma.pdf

What technologies do you use to study life on different planets?

So far scientists have not yet discovered any forms of life on other planets, but they do use a variety of methods to search for evidence of life on other planets. Within our own Solar System, scientists can send robotic rovers to take actual measurements of another planet’s geology. Mars exploration, for example, includes a search for chemical signatures in rocks or soil that could indicate present or past life. Scientists outfit these rovers with drills and scoops to collect samples, which are then fed into chambers that allow for the composition of the material to be analyzed according to its mass (using a mass spectrometer), its volatiles (using gas chromotography) or its laser-induced signature (using a laser spectrometer). The recent Mars Science Laboratory (named “Curiosity”, see: http://mars.jpl.nasa.gov/msl/) even includes an instrument that shoots a laser at a distant rock and observes its chemical composition from afar! Scientists compare the results of these chemical analyses with environments on Earth to understand whether or not these Martian environments are consistent with the presence of biology (for example, by looking for organics, carbon, amino acids, or other remnants of life). Scientists use these type of analyses to design better experiments that will ultimately help to conclude whether or not Mars has (or had) life. Similar exploration techniques are used, or will be used, in the exploration of other bodies within our Solar System.

Extrasolar planets present another challenge because they are too far away for even robotic exploration. Scientists must therefore rely on remote observations using telescopes to first identify extrasolar planets and then to characterize them according to their potential habitability. A telescope outfitted with a spectrometer can identify individual gaseous components of an extrasolar planet’s atmosphere, which provides at least some information about the planet. Using the past and present of Earth as a proxy, scientists think that certain combinations of atmospheric gases are likely due to the presence of life. The presence of oxygen, ozone, and methane, for example, is due to widespread life on Earth; if scientists were to observe an extrasolar planet with a similar composition, then it would be evidence for an inhabited Earth-like planet orbiting another star. Scientists have not yet discovered any such planets, but the telescopes to properly conduct this search are still in their infancy. As newer generations of telescopes come into being, we will eventually be able to conduct more careful searches that let us peer into the atmosphere of extrasolar planets—in an effort to find signs of life.

Life as we know it

It seems that members of the animal kingdom can live a plant-like existence (e.g. sponges), but you would not expect members of the plant or fungus family to evolve into something that could grow legs and walk around. So what is the fundamental difference between plants and animals, allowing animals to move?

Let’s consider this: moving behavior is not solely attributed to animals. We know not all plants are sedentary – consider, for example, slime molds, or choanoflagellates, which exhibit similar morphological properties to sponges.

Therefore, we cannot list moving as a fundamental distinctive property distinguishing plants from animals. To understand why some organisms are sedentary and some are not, we need to take the environment into consideration. Remember: environment selects!

Think about the lifestyle of a plant, for instance, and what it needs to survive: nutrients, sufficient soil, air and light. Therefore, it would make sense for a plant not to move in order to survive and to satisfy its basic needs. A plant does not have to actively pursue its food like animals must do.

We often see some closely related species exhibiting different properties. For those cases, we need to keep in mind that the divergence between these species may have occurred millions of years ago. Thus the organisms had a really long time to selectively adapt to their local environment.

Are there any living examples of LUCA (Last Universal Common Ancestor), or would it be possible to reconstruct one?

As far as we know, there are no living examples of the Last Universal Common Ancestor (LUCA) . It would certainly provide a lot of information about the early evolution of life.

Instead of finding a living sample of such ancient life, we’ve inferred the existence of LUCA by comparing DNA from all kinds different organisms alive today – the commonalities in the DNA of organisms as diverse as bacteria and humans suggest that all life on Earth is related (the fact that all known organisms on Earth have DNA is a powerful statement in its own right about common ancestry).

However, the phylogenetic record (the record of the history of life on Earth) that is preserved in DNA is really limited, in that we can only compare the DNA of organisms that exist today. Anything that went extinct in the past is not going to contribute to the DNA sequence data banks that we generate now. The information is simply lost from history. It’s only by using data from modern organisms that we’ve been able to figure out that a common ancestry relates all known life – at the base of the “tree” is LUCA.

BUT (and this is purposefully a big BUT) we don’t know what information might be missing about LUCA from all of the lineages that went extinct in the 3+ billion years since LUCA lived. So while it would be really cool if we could reconstruct all of the DNA of LUCA, and in principle it is definitely possible with modern technology as you suggest, it may not be possible in practice. We may have lost too much information over the course of evolutionary history to ever know exactly what LUCA looked like. The situation is so challenging when looking that far back in time (more than 3 billion years) that we are not even quite sure where to locate the root of the tree of life.

Known life is divided into three major domains: Eukaryotes, Bacteria, and Archaea. These three very different kinds of organisms are still related by common ancestry (that is, LUCA), but we don’t know which is most closely related to LUCA. Depending on how you construct your tree (i.e. which DNA sequences you look at) you end up with different relationships of the three domains near the root. So even though we know LUCA existed, there is lots more work to be done to understand what early life looked like.

Life as we don't know it

Why it is said that on another planet, if there is no oxygen or water then there is no possibility of life? Is it possible that life may exist on another planet that does not match with our concept of life?

Astrobiologists have not yet discovered any forms of life outside of Earth, and so Earth-life remains our only know example of possible biological systems. On Earth, all living organisms require liquid water during some part of their life cycle, which has suggested that one way of searching for extraterrestrial life is to “follow the water” in space. We do know of plenty of examples of life on Earth that does not require oxygen (known as anaerobic life), and astrobiologists do actively consider both oxygenated and oxygen-free environments when thinking about life elsewhere.

That said, some scientists study the possibility of life that could use other liquids—such as alcohol solvents—or other chemical backbones—such as arsenic or silicon, rather than carbon. We also cannot discount the possibility that extraterrestrial life might be so completely different from what we can imagine that it could be undetectable by our current search techniques. Until we discover an example of extraterrestrial life, we cannot known for certain, but thinking about the commonalities of Earth life is at least one step toward refining our search for life elsewhere.

It is said that dying red giant stars could act like a defibrillator and bring icy planets back from the dead & that this rebirth could also lead to new breeding grounds for life. How can new life survive without light?

While most life on Earth is powered by photosynthesis, it’s not the only way for an organism to survive. Organisms can get their energy from chemical reactions as well as sunlight, and these organisms are known as chemotrophs. There are actually a surprising number of ecosystems in places that never see the sun. The most famous of these, perhaps, are the communities found around hydrothermal vents, which are volcanic fissures in the ocean floor. Being on the bottom of the sea, they get essentially zero sunlight, but nonetheless, they host a thriving ecosystem, which is based on bacteria that “eat” the sulfur emitted by the vents, rather than on plants. If you want to learn more about these fascinatingly strange communities, Searching For Life Where the Sun Don’t Shine is a terrific five-part series of articles examining how things live without sunlight, and our efforts to learn more about them.

I know we are always looking for life in the universe, but what if life as we know it is really not the only life out there?

This is one of the biggest challenges that we face in astrobiology. We only have one example of life that we can study directly, but it would be pretty “Earth-centric” of us to assume that life elsewhere should look like life on Earth, or that it must be based on the same exact biochemistry.

However, as we learn more about how life on Earth works (at both the molecular and the organismal levels), we become better equipped to think about how life elsewhere may look and behave. We can think about how the principles of life on Earth could be extended in different ways. For example, are there other replicating polymers (we use DNA) that are made up of different chemical components? How many different classes of biomolecules might be required to get life started? What are the ranges of environmental conditions that can support life? These subjects are currently being researched by many labs within the NAI and elsewhere.

Of course, detecting non-Earth life is another huge challenge, since even if we point our sensors in the right direction, this life may generate different “biosignatures” than we use on Earth (for example, an oxygenated atmosphere). In this case, it might make sense to look for something that seems “out of place” in the atmosphere of another world. This is also a huge area of debate and research within the astrobiology community; as such, right now we are concentrating our search on worlds that should support water in its liquid state, which is necessary for life as we know it. It may be possible for life to exist in another solvent, and there is some research being done in this area, but we do not know yet for sure.

We have to combine both knowledge and creativity when we engage in astrobiology, which is why it is so important to have such a diverse group of scientists involved in the process.

If we were to ever find life “out there”, is there any way that this "life" could be made out any other elements such as silicone or metal. Is there any way we can make synthetic life/ DNA out of metal instead of sugar?

One of the reasons astrobiologists study life here on Earth is to understand the limits to life and understand whether life elsewhere in the Universe would be anything like we see here on our planet. So far, life as we know it is carbon- based. However, keep in mind this common definition for life – “a self-sustaining chemical system capable of Darwinian evolution.” Therefore, yes, life as we don’t know can be anything beyond our imagination, we just have to go and find it!

The second question is one of the main subject areas of synthetic biology. Various studies (such as those conducted by Steve Benner’s research group at the Foundation For Applied Molecular Evolution) aim to chemically synthesize genetic material made with alternative chemicals. Some researchers have been able to synthesize DNA and RNA analogs with simpler sugar backbones that can do the job just fine. The challenge for these molecules, however, has been for them to maintain their stability, required for undergoing Darwinian evolution and remaining biologically active. These experiments are mostly conducted under simulated Earth conditions. Understanding different environmental conditions on other planetary bodies will allow us to perform experiments that explore life’s potential beyond Earth.

What is the possibility that any life found beyond Earth will resemble life on our planet?

There are many theories about what kinds of life we may discover elsewhere in the Universe. Some scientists think life will be similar to us – based on carbon, oxygen, nitrogen and similar elements. Other scientists believe we may find life that is completely different – perhaps based on silicon instead of carbon, or able to use in liquid methane or ethane as a solvent, as opposed to liquid water. Based on a chemistry standpoint, I believe any life we find will probably be carbon-based, for several reasons. First, life that uses lighter elements like carbon and nitrogen would be more likely simply because these elements are more abundant in the Universe than heavier elements like silicon and arsenic. Second, we know that carbon-based compounds like methanol, carbon dioxide and amino acids are present on comets, in meteorites and in giant molecular clouds (interstellar clouds of gas in which the formation of molecules can take place), so they’re out there already. Third and most importantly, carbon-based life works on a thermodynamic level. In carbon-containing molecules, bonds to carbon atoms are just strong enough to be stable, but not too strong to be unbreakable, meaning that the molecules can store energy in those bonds and then access it later. This is how life on Earth can store energy from sunlight (if it’s a plant or photosynthesizing organism), or use the energy from sugars and other compounds in foods. Carbon just makes sense!

BUT, just because carbon makes sense, that doesn’t mean we won’t find weird life somewhere! We just need to keep our minds open to new possibilities and not overlook anything in our search for life.

Life in the Solar System

Is it possible that there is life on Uranus?

Life must be coupled to its environment, so let’s consider what we know about Uranus’ environment. Uranus is a gas giant like Jupiter, Saturn, and Neptune. These gas planets do not have an abrupt transition between a solid surface and an atmosphere like we have on Earth. The gaseous envelope cloaking Uranus, composed primarily of hydrogen and helium, would transition to an “ocean” as pressures increase further down into the planet. At depth, water, ammonia and methane exist as well and may form clouds. Uranus has the honor of having the coldest atmosphere of all the planets, measured at -371 degrees F. Temperatures increase below the atmosphere, probably reaching room temperature at depths where the pressure is about 200 times that of “surface” pressure. That’s the same pressure as we would feel under about 3,000 feet of water on Earth.

Pressure and temperature in the lower depths of Uranus are not extreme for life as we know it. Life forms have been found miles under the ocean’s surface on Earth. What is very different on Uranus is the lack of an adequate energy source for biology. Living in an atmosphere with no interaction with a rocky surface, where a lot of nutrients for life come from, is hard to imagine. While Uranus likely has a rocky core, the pressures and temperatures there would be such that life as we know it would be impossible. Higher up in the atmosphere of Uranus, where temperatures and pressures are not as extreme, life would not be impossible, though it still is improbable. Keep in mind, however, that astrobiologists are still learning about how incredible Earth biology is at adapting to extreme environments.

In Ben Bova’s science fiction book “Jupiter”, humans plunge into Jupiter and find “balloon-like” life-forms living within the dense gas environment of the planet, and they communicate with light signals. Is life impossible on Uranus? No. Is it improbable based on what we know about carbon-based life? Yes.

If irrefutable evidence of life was found on Mars, for example fossils or evidence of current life under the surface, how important would this be, and how would this affect future NASA space missions?

Evidence for life on Mars would affect everyone in different ways. Some may not find it important at all, some may find it threatening for religious reasons, and some may feel this would be the most important discovery ever made. The discovery of Life on Mars would certainly bring up some interesting questions from a scientific perspective. The most important step would be determining the origin of that life, and if it shared a common ancestor with Earth. If it is related to Earth-life, the next step (if possible) would be determining who seeded who. Did life originate on Earth and then seed Mars via meteorite impact? Or did life originate on Mars and seed Earth with life? If this life were of extraterrestrial origin it would certainly turn more than a few heads!

Astrobiology is the study of Life in the Universe, and as far as this field is concerned, finding a type of life that did not originate from Earth would mean our data set, of one, has now doubled. If you think about it, everything that we know about Life in the Universe came from Earth. If we had another sample of life – life that found a different way to evolve – the potential of gaining a greater understanding of the Life in the Universe is immense! A discovery like this would give hope that it’s not just us out there in Cosmos. I would speculate that NASA’s funding priorities would be altered to study this evidence of life in great depth. Depending on funding and current technology available, further steps could include funding more specialized rovers/landers, funding a sample return mission, or even as bold as funding a manned mission to Mars. Let us hope in the future, a discovery like this would bring nations together to collaborate on funding and research, as any of the mentioned missions would be a heavy financial burden on any one country.

Careers in Astrobiology

How can I get a job as an astrobiologist at NASA?

Most jobs in astrobiology are at universities or in industry, not working directly for NASA. Almost all the teams who are part of the NASA Astrobiology Institute are in academic organizations. Thus your best bet is not to apply directly to NASA but to go to graduate school at one of the universities that offer courses in astrobiology, or else to apply directly to universities or other labs that are carrying out research in astrobiology.

I'm interested in becoming an astrobiologist. What classes should be taken in college and what are the best schools to go to in this field?

The field of astrobiology is relatively new when compared to the long established fields of astronomy, biology, physics, geology, planetary science, etc. At this time, there are very few dedicated degree programs in astrobiology. The typical pathway for a student interested in pursuing astrobiology graduate studies is to specialize in a single scientific discipline. You should choose a field that really excites you. This discipline will be the base of knowledge that you will build upon, so make it one you are passionate about.

Undergraduate Studies: As you work on building a foundation in your field of interest, educate yourself about astrobiology. There are many good Astrobiology Primer is a good reference tool to understand, at least at a fundamental level, the field of astrobiology. You can browse this NASA Astrobiology Program website for the latest information on astrobiology careers, education, funding, news, and publications. You should sign up to receive the Astrobiology Newsletter (on the bottom of any page on this website) to keep current on astrobiology happenings.

Graduate Studies: When looking for graduate schools, look at research topics of individual scientists involved in the part of astrobiology you are interested in, and focus your graduate applications towards working with those individuals.

A good place to start your search is by looking at the research projects within the NASA Astrobiology Program as well as the NASA Astrobiology Institute Teams and Annual Reports.

Postdoctoral Work: Currently, PhD’s are not awarded solely in astrobiology. As you look towards your postdoctoral work you will want to identify a research group involved in the research you are interested in, whether that’s searching for exoplanets or understanding microbes in extreme environments. Again, you can look through NASA Astrobiology Program research projects, NASA Astrobiology Institute Teams and Annual Reports. You should read recently published papers from the group you’re interested in and visit their website to gain an appreciation for what they do and how that fits with your interests. Then contact the Principal Investigator (PI) to see if they have Ph.D. positions available.

Where are organic compounds usually found in the Solar System and where do they form?

Organics can form in young galactic areas and be delivered to solar systems. (In the question’s context, it is assumed that “organic compounds” means those with carbon in them, that often resemble or even are the same as compounds used or made by life.)

Some scientists use the method of spectroscopy – analysis of the way light is absorbed and emitted by objects – to study organics in nebulae (huge star-forming clouds of dust and gas the size of galaxies). They have identified organic compounds in these parts of the universe, and they’ve developed some theories as to how they form. The organics are formed in circumstellar areas, where young stars are forming, and escape with other interstellar dust particles into the interstellar medium. They may get sucked into nearby planetary nebulae, which are areas where planets are forming. The formation of stars or planets can incorporate those organic compounds.

Meteorites, which are sort of like “crumbs” left over from planet formation, can also carry organic compounds through space. When a meteorite strikes a planet, it can deposit those organics. Some scientists theorize that this deposition process may have played a role in the origin of how life on Earth.

The discovery of amino acids and carbon compounds in comets and the current speculation that comets delivered these necessary ingredients for life to a "barren" Earth prompts the question, How did amino acids and the other complex compounds found in cometary bodies get there and where did all the water that comprises the majority of the ort cloud come from?

The water ice in the Oort Cloud, like almost all of the material in the solar system, came originally from a large interstellar cloud of gas and dust called the presolar nebula. Due to the particular mixture of elements injected into the presolar nebula from earlier stars, there was more oxygen than carbon or nitrogen but far more hydrogen than anything else. This produced a large amount of water, carbon dioxide, carbon monoxide, ammonia, and methane – what astronomers call ices or volatiles. As the cloud collapsed to form the solar system, it formed a disc around the young Sun. The inner parts of the disc (out to what would eventually be the inner asteroid belt) were hot enough that those compounds were gases rather than solids, so the planetesimals that later formed the terrestrial planets only had relatively little ices in them. But the outer parts of the disc were cold enough for water to be a solid, with CO2, CO, NH3 and CH4 becoming solid even further out. That’s why the objects in the Oort Cloud and the giant planets and their satellites have large quantities of water and other ices in them.

The amino acids and other carbon compounds in meteorites and comets formed as those ices reacted with each other, and the smaller quantities of many other compounds that were produced in the presolar nebula and before that in the stellar winds of earlier stars. These were then later delivered to the Earth as the orbits of comets and asteroids were perturbed to bring them into the inner solar system. This still happens, but much of the material that was delivered to Earth arrived during a time 3.8 billion years ago called the Late Heavy Bombardment, caused by the orbits of the giant planets changing rapidly.